Thesis (DPhil (Electrical and Electronic Engineering))--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: The Karoo Array Telescope (KAT) will be used by South Africa in its bid to host the
international Square Kilometre Array (SKA). As the SKA will have orders of magnitude greater
sensitivity than existing radio telescopes, it will also be concomitantly more sensitive to radio
frequency interference (RFI). The influence of RFI on a differential mode (DM) KAT
conductive system is an unavoidable phenomenon. In this context, the conductive or galvanic
system can, in its most basic form, consist of a source, a load, and connecting conductors. It can
also, in the case of the KAT-7 seven-dish interferometer, consist of each telescope, its functional
cabling, and the main correlator connecting the telescopes together. However, additional
connections between the system and the environment exist. These might be an intentional
connection made to the earthing layout, or unintentional connections due to parasitic
capacitances and inductive connections. As a result of this, additional conductive systems are
created which carry common mode (CM) currents. Interference present in such CM paths enters
the DM system through the transfer impedance (Zt) between them. To reduce or eliminate this
interference in the DM system, Zt has to be minimised.
The use of an earthed parallel conductor (EPC as commonly referred to) in the form of a cable
tray is considered to be one of the principal methods to reduce Zt. The properties of cable trays as
EPCs at wavelengths which are greater than the tray length are well documented. One main
focus in this dissertation is on cable tray mid-span and end connections. They are not well
described in the literature over the wide range of frequencies that is expected for KAT
developments. The influence of the most common connections on the measured and computed Zt
of the cable tray is determined. Computer Simulation Technology’s Microwave Studio (CST
MWS) is employed to validate the measured results and also to enable visualisation of the fields
and currents. Recommendations for the best connection to use for any cable tray installation is
given. The overall shielding ability of optimally-connected cable trays has been evaluated using
a physical and computational model. In both cases the induced voltage on a victim conductor,
and far-field gain functions for varying angles of incidence onto the model, are determined. The
results also show that for certain scenarios, most of the coupling to the victim conductor, takes
place inside the end enclosure and not the cable tray. In general, properly-connected cable trays
do provide protection to their enclosed conductors, even at frequencies were the wavelength is
much shorter than the width of the tray. The second main focus arises from an on-site radio frequency (RF) current audit undertaken on
two of the seven KAT-7 telescopes. Shielding measures, such as interface barriers at the floor
and roof of the lower telescope pedestal, are studied. The investigation is facilitated by the
development of an accurate physical and computational scale model of the dish. Direct current
injection and plane wave illumination methods are used to excite the system. The measured CM
current distributions are compared and comments made regarding the validity of the
measurement procedure. The CM currents, measured around the outside of the lower pedestal
show higher levels when a direct current path to ground is established, as opposed to when no
clear path exists. This finding suggests at least two methods of preventing CM interference
entering or leaving the pedestal: harden the floor and roof barriers, or manage current paths
outside the telescope. Related to this, CM currents measured either side of the telescope interface
barriers, are used to determine the level of shielding the interface provides. When compared to
the common definition of shielding effectiveness, the current measurement provides more
conservative shielding estimates.
The research in this dissertation has influenced, and will continue to influence, the layout of
galvanic systems for the present KAT-7 structures and the anticipated developments to
MeerKAT. / AFRIKAANSE OPSOMMING: Die Karoo Array Telescop (KAT) sal gebruik word deur Suid-Afrika in sy poging om die bod te
kry om die internasionale Square Kilometre Array (SKA) te huisves. Aangesien die SKA ordes
meer sensitief sal wees as bestaande radioteleskope, sal dit terselfdertyd ook meer sensitief wees
vir radio frekwensie steurnisse. Die invloed van radio frekwensie steurnisse op die
differentiëlemodus KAT geleidende netwerk is iets onvermydeliks. Binne hierdie konteks kan
die geleidende netwerk, in sy mees basiese vorm, bestaan uit ʼn bron, ʼn las, en
verbindingsgeleiers. Dit kan ook, in die geval van die KAT-7 sewe-teleskoop interferometer,
bestaan uit elke teleskoop, sy funksionele bekabeling, en die korrelator wat die verbinding is
tussen al die teleskope. Daar is egter, addisionele verbindings tussen die netwerk en die
onmiddellike omgewing rondom dit. Hierdie verbindings kan opsetlik gemaak word deur
byvoorbeeld, verbinding van die netwerk se aardkabel aan die res van die beaarding, of
onopsetlik deur parasitiese kapasitansies en induktiewe verbindings. As gevolg hiervan word
addisionele geledende netwerke geskep waarin gemenemodus strome kan vloei. Indien daar wel
strome in die gemenemodus netwerk vloei, word dit oorgedra aan die differentiëlemodus
netwerk deur ʼn oordragimpedansie (Zt) wat teenwoordig is tussen die twee. Om dus die
steurnisse in die differentiëlemodus netwerk te verminder of te elimineer, moet Zt tot ʼn minimum
beperk word.
Die gebruik van ʼn geaarde parallelle geleier in die vorm van ʼn kabelkanaal, word beskou as een
van die mees doeltreffendste metodes om Zt te verminder. Die eienskappe van kabelkanale as
geaarde parallelle geleiers by frekwensies waar die golflengtes langer is as die van die
kabelkanaal, is volledig gedokumenteer. Een van die belangrikste fokuspunte in hierdie
verhandeling is rakende die kabelkanale se middel en eindpuntverbindings. Hulle word nie goed
beskryf in die beskikbare literatuur nie, en weliswaar vir wyeband doeleindes wat vir KAT
ontwikkelinge verwag word. Die invloed van die mees algemeenste kabelkanaal verbindings op
gemete en berekende Zt word bepaal. Computer Simulation Technology’s Microwave Studio
(CST MWS) word eerstens, gebruik om die akkuraatheid van die gemete resultate te bewys en
tweedens, deur visualisering van E-veld en gemenemodus oppervlak strome. Aanbevelings vir
die beste verbindings vir enige kabelkanaal opstelling word gegee. Die algemene
afskerminsvermoeë van ʼn idiaal-verbinde kabelkanaal word bepaal deur middel van metings en simulasies. In beide gevalle word die geïnduseerde spanning op ʼn slagoffer kabel, en die verveld
aanwins funksie bepaal vir verskillende invalshoeke op die model. Die resultate toon verder
dat vir spesifieke gevalle wat beskou word, die meeste koppeling binne die kabinet aan die einde
van die kabelkanaal plaasvind. Oor die algemeen verskaf goed verbinde kabelkanale wel ʼn
sekere vlak van beskerming aan kabels binne die kabelkanaal, selfs by frekwensies waar die
golflengte baie korter is as die breedte van die kabelkanaal.
Die tweede belangrike fokuspunt spruit voort uit ʼn radio frekwensie stroomoudit, wat twee van
die KAT-7 teleskope evalueer het. Afskermingsmatreëls soos die kabelhindernisse op die vloer
en dak van die onderste teleskoop voetstuk, word bestudeer. Tesame met die metings op die
werklike teleskoop wat geneem is, word ʼn akkurate fisiese en simulasie skaalmodel geskep om
die metings beter te analiseer. Direkte stroominspuitings metode en platvlakgolf beligting word
gebruik om gemenemodus strome op die teleskoop se struktuur te induseer. Die gemenemodus
stroomverspreiding vir beide tegnieke word vergelyk in ʼn poging om kommentaar te lewer
rakende die geldigheid van die meettegniek. Die gemenemodus strome wat aan die buitekant
van die teleskoop voetstuk gemeet word, is hoër wanneer ʼn direkte stroompad na grond op die
voetstuk geskep word in vergelyking met ʼn ongedefinieerde pad. Hierdie verskynsel dui daarop
dat ten minste twee metodes bestaan om die ongevraagde gemenemodus strome te verhoed om
aan die binnekant van die teleskoop voetstuk te vloei. Die een is die verbetering van die vloer en
dak kabelhindernisse, en die ander is verbetering van die stroompad (stroompaaie) aan die
buitekant van die voetstuk sodat ʼn meer direkte pad na grond geskep word. Die gemiddelde
gemenemodus strome weerskante van die kabelhindernis, kan gebruik word op te bepaal hoeveel
afskerming die hindernis bied. Indien die berekende waardes by verskillende frekwensies
vergelyk word met die tradisionele filter doeltreffendheid (Zt), word ʼn meer konserwatiewe
beraming verkry.
Die navorsing in hierdie verhandeling het alreeds, maar sal ook die toekomstige uitleg van
galvaniese stelsels vir KAT-7 asook die verwagte MeerKAT beïnvloed.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/6559 |
| Date | 03 1900 |
| Creators | Van der Merwe, Paul Stephanus |
| Contributors | Reader, H. C., University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 183 p. : ill. |
| Rights | University of Stellenbosch |
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